Positron Emission Tomography is an “in vivo” diagnostic and research technique for imaging, capable of measuring the metabolic activity of the human body. The PET technique is based on detecting and analyzing the three-dimensional distribution that an ultrashort half-life radiopharmaceutical, administered through an intravenous injection, takes inside the body. Depending on what you want to study, different radiopharmaceuticals are used.
The image is obtained thanks to the fact that the devices are able to detect the gamma photons emitted by the patient. These 511 keV gamma photons are the product of an annihilation between a positron, emitted by the radiopharmaceutical, and a cortical electron from the patient's body. This annihilation gives rise to the emission, essentially, of two photons. For these photons to end up shaping the image they must be detected “in coincidence”, that is, at the same time, in an appropriate time period (nanoseconds).
In addition, they must come from the same direction and opposite sense of direction, but also their energy must exceed a minimum threshold that certifies that they have not suffered significant energy dispersions in their journey (scatter phenomenon) to the detectors. The detectors of a PET scanner are arranged in a ring-shaped structure around the patient, and because they detect in coincidence the photons generated in each annihilation, they will make up the image. To obtain the image, these detected photons are converted into electrical signals. This information is then subjected to filtering and reconstruction processes, thanks to which the image is obtained.
The dedicated brain PET is useful for the measurement of brain activity and is effective for the early diagnosis of neurodegenerative diseases such as Parkinson's disease or Alzheimer's disease, as well as other mental illnesses such as schizophrenia or severe depression. For an accurate diagnosis high quality images are required, therefore, the device must be designed with high spatial resolution and sensitivity. The sensitivity can be improved by increasing the thickness of the crystals, decreasing the distance of the detector to the patient and/or covering the maximum possible surface of the patient's skull.
Various approaches have been proposed to solve the problem of improving the sensitivity of brain PET, such as patent application WO2010/033159, where Majewski et al. They propose a simple spherical ring around the head to generate the image. The invention described in this application has the disadvantages that the ring does not cover the entire brain and, having a circular shape, is not optimized for the typically oval shape of the human head. Likewise, in 2011, the article by S. Yamamoto et al. is published in the IEEE Transactions on Nuclear Science (vol. 58, pp. 668 to 673). “Development of a Brain PET system, PET-Hat: A Wearable PET System for Brain Research” where an equally circular and single-ring PET device is described, showing no advance in the aspect of sensitivity with respect to the aforementioned patent. In 2013, Weinberg I. et al described in the patent US2013218010 a multi-ring device of circular section that includes partial rings of detectors, which do not complete the ring, in order to increase the sensitivity. All these works have in common that they are based on rings of circular section using square detectors.
In 2015, Tashima et al. describe in the U.S. Pat. No. 9,226,717 B2 (US201501 15162 A1) a PET device, of similarly circular section, but organized in the form of a hemisphere instead of a cylindrical one, which incorporates an element not physically coupled to the main hull in order to increase its sensitivity.
The construction of a device that optimizes the sensitivity of a dedicated brain PET, which at the same time minimizes the number of detectors used, would require building a surface that would be completely adapted, in shape and size, to the head, particularly the human head. However, there are important limitations as to how to generate this surface due to the procedure used to manufacture the continuous scintillating crystals that are included in these devices. These limitations are related to the maximum size and shape in which these crystals can be carved.
Further, it is impossible to perform exactly a three-dimensional elongated curved surface (such as an ellipsoid) starting from flat surfaces in the form of polygons. However, although it is not trivial, it is possible to approximate those curved surfaces to a polyhedron constructed from flat surfaces in the form of polygons. The object of the present invention is precisely to achieve a PET imaging device with the maximum angular coverage of the brain by means of independent detection modules of polygonal main section and together constituting a three-dimensional elongated structure adapted to the head, in particular the human head and with ability to be arranged as close as possible to the head, to minimize the number of detectors.
Taking into account the limitations existing in the manufacturing of crystals, and using the regular geometric shapes previously proposed in the state of the art, the device is either too far from the actual geometry of the head (as seen in FIG. 1), or the truncated icosahedron that illustrates the state of the art), or too small due to the number of faces and maximum size of each face (typically 70 mm), as in the case of the truncated icosahedron.